In the last decade, environmental, industrial, and military monitoring applications have motivated research related to wireless sensing. Some applications, such as structural health monitoring, would benefit from sensing that is deeply embedded in the environment for an extended period of time. For such applications, it is desirable that the sensing device has an indefinite operating lifetime and a compact geometry for deployment ease and to ensure that the integrity of the structure being monitored is not compromised. The former constraint necessitates either that the sensing device has sufficient on-board energy resources to sustain an active device for the deployment lifetime or that the node be passive. For use in civil infrastructure, the approach of using an on-board energy resource may not be tenable. Because of this, there has been a large amount of recent work on passive wireless sensing systems.
In addition to on-board direct current (DC) power considerations, a passive device should require very low externally provided power for activation to ensure sufficient wireless transmission range. One approach for providing such power is through radio frequency (RF) interrogation. Both passive radio frequency identification (RFID) and surface acoustic wave (SAW) sensors operate on this premise. Unfortunately, such devices are typically constrained to short-range implementations, typically a few feet, because of their requisite activation power.
In view of the above discussion, it can be appreciated that it would be desirable to have a compact passive device for wireless sensing that requires relatively low externally provided power for activation and that provides sufficient wireless transmission range.